Why do humans have such enormous brains for our size? One popular explanation is that we needed the added cognitive capacity to deal with large, complex social groups. And now, a fascinating new computer simulation suggests that cooperation really did make us smarter.

A team of researchers at Trinity College Dublin simulated fifty very simple brains, each with only three to six neurons. The brains then faced each other in a pair of classic psychology games, the Prisoner's Dilemma and the Snowdrift Game. In both instances, doing the helpful thing can backfire if the other person betrays them — whether it's refusing to implicate the other person in a crime or volunteering to dig them out of a snowbank — but both sides can achieve acceptable results if they cooperate. Choosing to cooperate is only a really good strategy if you have played enough times to know whether your partner is trustworthy or not.

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The researchers used these games as a very simple proxy for the cooperative behaviors we see among humans and our primate cousins, but the results were still intriguing. ScienceNOW reports:

After playing one of the games, the brains reproduced asexually. Individuals that did better were programmed to be more likely to have offspring. Then all of the brains in the new generation had a chance to undergo a random mutation. The mutations could change the brain's structure, number of neurons, or the strengths of the connections between those neurons. Each simulation ran for 50,000 generations, with 10 runs of the simulation for each of the two games.

As time went on, the researchers measured how much the brains cooperated with each other and how many neurons the brains had-an indicator of how intelligent they were...Bigger brains did better as cooperation increased. That meant they got to reproduce more, which meant more brains had the capacity to cooperate with others.

The researchers aren't suggesting that this model has actually proven that social cooperation is what made bigger brains evolve. However, they do argue that the presence of such cooperative behaviors should be enough for intelligence to evolve, even if the story of primate evolution is almost certainly more complex than that. The social intelligence hypothesis probably doesn't explain everything about our evolutionary past, but it's likely a key piece of the puzzle.